Nitrile group-containing highly saturated copolymer rubber and rubber composition having improved cold resistance

ABSTRACT

A nitrile group-containing, highly saturated copolymer rubber is disclosed, the copolymer chain of which is comprised of (1) 5 to 40% by weight of units of an unsaturated nitrile monomer, (2) 1 to 80% by weight of units of a monomer selected from a fluorine-free unsaturated carboxylic acid ester monomer and a fluorine-containing vinyl monomer and (3) up to 20% by weight of units of a conjugated diene monomer, with the balance being (4) units of a hydrogenated conjugated diene monomer, wherein the sum of the contents of the monomer units (1) and (2) is 30 to 90% and the sum of the contents of the monomer units (3) and (4) is 10 to 70% by weight. This copolymer rubber results in a rubber composition having an improved cold resistance.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a rubber composition having an improvedcold resistance, and comprising, as a rubber component, a nitrilegroupcontaining copolymer having a specific composition and a reducedamount of an unsaturated bond.

(2) Description of the Related Art

When used for a control of automobile exhaust emissions, rubber partsdisposed in the vicinity of an engine must have a high heat resistance.To meet this demand, a nitrile group-containing, highly saturatedcopolymer rubber formed by hydrogenating a part or all ofcarbon-to-carbon double bond-containing monomer units in a nitrilegroup-containing hydrocarbon rubber such as an acrylonitrile/butadienecopolymer rubber (hereinafter referred to as "NBR") was developed and isnow in use (see U.S. Pat. No. 4,404,329).

This rubber has an ozone resistance and heat resistance superior tothose of conventional NBR, but has a cold resistance inferior to that ofNBR in some cases depending upon the nitrile group content or theunsaturation degree of the main chain of the polymer.

Due to recent advances in this field, the range of temperatures of theatmospheres in which various rubber parts are used has been extended tolower and higher temperatures and therefore, the development of anitrile group-containing, highly saturated copolymer rubber compositionhaving an improved cold resistance is urgently required in this field.

In a conventional NBR, a reduction of the nitrile group content willimprove the cold resistance. For example, in the TR test specified inASTM D-1329 (an elongated test piece is frozen, the temperature isgradually elevated, and the recovery of the elongated test piece ismeasured: more specifically, the temperature at which the length of thetest piece is contracted, i.e., recovered, by 10% by elevation of thetemperature is designated as TR10), NBR having a bonded acrylonitrilecontent of 37% by weight shows a TR10 value of -20.5° C., but if thebonded acrylonitrile content is reduced to 28% by weight, the TR10 valueis lowered to -31° C.

Nevertheless, a nitrile group-containing, highly saturated copolymerrubber is different from NBR in that, even if the nitrile group contentis reduced, the cold resistance is not always improved. For example, theTR10 of a rubber comprising 5% by weight of butadiene units and 37% byweight of acrylonitrile units, with the balance being hydrogenatedbutadiene units, is -24.5° C. but the TR10 of a rubber comprising 5% byweight of butadiene units and 28% by weight of acrylonitrile units, withthe balance being hydrogenated butadiene units, is -20° C., and the coldresistance is not improved even if the acrylonitrile content is reduced.

SUMMARY OF THE INVENTION

Therefore, a primary object of the present invention is to provide anitrile group-containing, highly saturated copolymer rubber having animproved cold resistance, and a composition comprising this copolymerrubber.

More specifically, in accordance with one aspect of the presentinvention, there is provided a nitrile group-containing, highlysaturated copolymer rubber the copolymer chain of which comprises (1) 5to 40% by weight of units of an unsaturated nitrile monomer, (2) 1 to80% by weight of units of a monomer selected from a fluorine-freeunsaturated carboxylic acid ester and a fluorine-containing vinylmonomer, and (3) up to 20% by weight of units of a conjugated dienemonomer, with the balance being (4) units of a hydrogenated conjugateddiene monomer, wherein the sum of the contents of the monomer units (1)and monomer units (2) is 30% to 90% by weight and the sum of thecontents of the monomer units (3) and monomer units (4) is 10 to 70% byweight.

In accordance with another aspect of the present invention, there isprovided a rubber composition having an improved cold resistance, whichcomprises a nitrile group-containing, highly saturated copolymer rubberas set forth above, and additives.

When the rubber composition of the present invention is used, a curedproduct (comprising 40 parts by weight of SRF carbon black and 100 partsby weight of the rubber without a plasticizer) having a TR10 lower than-26° C., which is not attainable when the butadiene units of NBR arehighly hydrogenated, can be prepared.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

and cold

From the viewpoint of the oil resistance resistance, in the nitrilegroup-containing, highly saturated copolymer of the present invention,the content of the nitrile group-containing monomer units (1) is 5 to40% by weight, preferably 10 to 35% by weight. If the content of themonomer units (1) is lower than 5% by weight, the oil resistance ispoor, and if the content of the monomer units (1) exceeds 40% by weight,there is little improvement of the cold resistance.

The content of the units (2) of the fluorine-free unsaturated carboxylicacid ester monomer and or the fluorine-containing vinyl monomer is 1 to80% by weight preferably 15 to 60% by weight. If the content of themonomer units (2) is lower than 1% by weight, there is littleimprovement of the cold resistance and if the content of the monomerunits (2) exceeds 80% by weight, the cold resistance is improved but theoil resistance becomes poor.

From the viewpoint of the heat resistance, the content of the conjugateddiene monomer units (3) in the copolymer rubber is up to 20% by weight,preferably up to 15% by weight. The content other than the content ofthe monomer units (1), (2) and (3) is composed of the hydrogenatedconjugated diene monomer units (4).

Moreover, to maintain a good balance between the oil resistance and coldresistance, the sum of the contents of the monomer units (1) and (2) is30 to 90% by weight, preferably 40 to 90% by weight. Surprisingly, thecold resistance is not improved outside this range. Furthermore, tomaintain a good balance between the heat resistance and cold resistance,the sum of the contents of the monomer units (3) and (4) is 10 to 70% byweight, preferably 10 to 60% by weight.

The content of the monomer units (2), the sum of the contents of themonomer units (1) and (2) and the sum of the contents of the monomerunits (3) and (4), which are desired for the improvement of the coldresistance, vary depending upon the particular fluorinefree unsaturatedcarboxylic acid ester monomer and/or the particular fluorine-containingvinyl monomer. More specifically, where a fluorine-free unsaturatedcarboxylic acid ester is used as the monomer for forming the monomerunits(2), it is preferable that the content of the monomer units (2) is15 to 60% by weight, the sum of the contents of the monomer units (1)and (2) is 40 to 90% by weight, more preferably 55 to 90% by weight, andthe sum of the contents of the monomer units (3) and (4) is 10 to 60% byweight, more preferably 10 to 45 by weight. Where a fluorine-containingvinyl monomer is used as the monomer for forming the monomer units (2),it is preferable that the content of the monomer units (2) is 5 to 60%by weight, the sum of the contents of the monomer units (1) and (2) is40 to 90% by weight, and the sum of the contents of the monomer units(3) and (4) is 10 to 60% by weight.

The nitrile group-containing, highly saturated copolymer rubber of thepresent invention is obtained by hydrogenating conjugated monomer unitsin a copolymer rubber, formed by copolymerizing (1) an unsaturatednitrile monomer, (2) at least one monomer selected from fluorine-freeunsaturated carboxylic acid esters and fluorine-containing vinylmonomers, and (3) a conjugated diene monomer by a conventional method.

The monomers to be used for the production of the copolymer rubber ofthe present invention will now be described.

As the unsaturated nitrile monomer, there can be mentioned, for example,acrylonitrile, methacrylonitrile and α-chloroacrylonitrile.

As the fluorine-free unsaturated carboxylic acid ester monomer, therecan be mentioned, for example, alkyl acrylates and methacrylates having1 to 18 carbon atoms in the alkyl group, such as methyl acrylate, ethylacrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentylacrylate, n-hexyl acrylate, 2-methylpentyl acrylate, n-octyl acrylate,2-ethylhexyl acrylate, n-dodecyl acrylate, methyl methacrylate and ethylmethacrylate; alkoxyalkyl acrylates having 2 to 12 carbon atoms in thealkoxyalkyl group, such as methoxymethyl acrylate, methoxyethylacrylate, ethoxyethyl acrylate, butoxyethyl acrylate, ethoxy-propylacrylate, methoxyethoxy acrylate and ethoxybutoxy acrylate; cyanoalkylacrylates having 2 to about 12 carbon atoms in the cyanoalkyl group,such as α-and β-cyanoethyl acrylates, α, β- and γ-cyanopropyl acrylates,cyanobutyl acrylate, cyanohexyl acrylate and cyano-octyl acrylate;hydroxyalkyl acrylates having 1 to 18 carbon atoms in the hydroxyalkylgroup such as 2-hydroxyethyl acrylate and hydroxypropyl acrylate;aminoalkyl esters of ethylenically unsaturated carboxylic acids, having1 to 12 carbon atoms in the aminoalkyl group, such as methylaminoethylacrylate, t-butylaminoethyl acrylate, dimethylaminoethyl acrylate,dimethylaminopropyl acrylate, diethylaminoethyl acrylate,dibutylaminoethyl acrylate, methylaminoethyl methacrylate,t-butylaminoethyl methacrylate, dimethylaminoethyl methacrylate,dimethylaminopropyl acrylate, diethylaminoethyl methacrylate anddibutylaminoethyl methacrylate; and mono- and di-alkyl esters ofunsaturated carboxylic acids, having 1 to 8 carbon atoms in the alkylgroup, such as monoethyl maleate, dimethyl maleate, dimethyl fumarate,diethyl fumarate, di-n-butyl fumarate, di-2-ethylhexyl fumarate,dimethyl itaconate, and di-n-butyl itaconate.

Of these fluorine-free unsaturated carboxylic acid ester monomers, alkylacrylates and methacrylates having 4 to 12 carbon atoms in the alkylgroup, aminoalkyl acrylates and methacrylates having 1 to 12 carbonatoms in the aminoalkyl group, and hydroxyalkyl acrylates having 1 to 18carbon atoms in the hydroxyalkyl group are preferable.

As the fluorine-containing vinyl monomer, there can be mentioned, forexample, fluoroalkyl acrylates and methacrylates having 3 to 21 fluorineatoms in the fluoroalkyl group, such as trifluoroethyl acrylate,tetrafluoropropyl acrylate, pentafluoropropyl acrylate, heptafluorobutylacrylate, octafluoropentyl acrylate, nonafluoropentyl acrylate,undecafluorohexyl acrylate, pentadecafluoro-octyl acrylate,heptadecafluorononyl acrylate, heptadecafluorodecyl acrylate,nonadecafluorodecyl acrylate, trifluoroethyl methacrylate,tetrafluoropropyl methacrylate, octafluoropentyl methacrylate,dodecafluoroheptyl methacrylate, pentadecafluoro-octyl acrylate, andhexadecafluorononyl methacrylate; fluorine-substituted benzyl acrylatesand methacrylates such as fluorobenzyl acrylate, difluorobenzylacrylate, fluorobenzyl methacrylate and difluorobenzyl methacrylate;fluoroalkyl vinyl ethers having 1 to 8 carbon atoms in the fluoroalkylgroup, such as fluoroethyl vinyl ether, fluoropropyl vinyl ether,trifluoromethyl vinyl ether, trifluoroethyl vinyl ether, perfluoropropylvinyl ether and perfluorohexyl vinyl ether; and o- andp-trifluoromethylstyrenes, vinyl pentafluorobenzoate, difluoroethyleneand tetrafluoroethylene. Of these fluorine-containing vinyl monomersfluoroalkyl acrylates and methacrylates having 3 to 21 fluorine atoms inthe fluoroalkyl group are preferable.

As the conjugated diene monomer, there can be mentioned, for example,1,3-butadiene, 2,3-dimethylbutadiene, isoprene and 1,3-pentadiene.

Parts of the above-mentioned monomers can be substituted by monomerscopolymerizable with the above-mentioned monomers, for example, vinylmonomers such as styrene and vinylpyridine and non-conjugated dienemonomers such as vinylnorbornene, dicyclopentadiene and 1,4-hexadiene,so long as the intended object of the present invention can be attained.

The nitrile group-containing, highly saturated copolymer rubber of thepresent invention is obtained by copolymerizing the above-mentionedmonomers by customary polymerization procedures, and then hydrogenatingthe conjugated diene monomer units of the thus-obtained copolymerrubber.

For example, there can be mentioned copolymer rubbers obtained byhydrogenating the butadiene units and isoprene units of copolymerrubbers such as a butadiene/butyl acrylate/acrylonitrile copolymerrubber, a butadiene/isoprene/butyl acrylate/acrylonitrile copolymerrubber, a butadiene/2-ethylhexyl acrylate/acrylonitrile copolymerrubber, a butadiene/trifluoroethyl acrylate/acrylonitrile copolymerrubber, a butadiene/trifluoroethyl methacrylate/acrylonitrile copolymerrubber, a butadiene/diethylaminoethyl methacrylate/acrylonitrilecopolymer rubber and a butadiene/di-n-butyl itaconate/acrylonitrilecopolymer rubber.

A rubber composition of the present invention is prepared by mixing thenitrile group-containing, highly saturated copolymer rubber with variousadditives customarily used in the rubber industry in a conventionalmixer. The kinds and amounts incorporated of the additives aredetermined in accordance with the intended object (use) of the rubbercomposition. Usually, the rubber composition comprises, based on theweight of the rubber composition, 25 to 90% by weight of the nitrilegroup-containing, highly saturated copolymer rubber of claim and 75 to10% by weight of additives.

As the additives, there can be mentioned sulfurcuring systems comprisinga sulfur donor compound such as sulfur or tetramethylthiuram disulfide,zinc oxide, stearic acid and a curing promoter of the guanidine,thiazole, thiuram or dithiosulfate type; organic peroxide-curing systemscomprising an organic peroxide such as dicumyl peroxide or2,5-dimethyl-2,5-di(t-butyl-peroxy)hexine-33 and a cross-linking agentsuch as triallyl cyanurate or trimethylolpropane trimethacrylate; carbonblacks of various grades such as SRF, HAF and FEA, reinforcers such assilica, talc and calcium carbonate, fillers, plasticizers, process oils,processing assistants, and aging-preventing agents.

By using the rubber composition of the present invention, a curedproduct having a TR10 lower than -26° C., which is not attainable by acomposition comprising a conventional highly saturated NBR, can beprepared (40 parts by weight of SRF carbon black and 100 parts by weightof the rubber; a plasticizer is not incorporated).

The rubber composition of the present invention has the characteristicsof the nitrile group-containing, highly saturated copolymer rubber, suchas a high ozone resistance, high heat resistance, and high oilresistance, and further, the rubber composition has an excellent coldresistance. Accordingly, the rubber composition of the present inventionis effective for use in the production of rubber products which are incontact with an oil or gas during use, and for which a high heatresistance and oil resistance are required, especially rubber productsfor which a high cold resistance is required.

The rubber composition of the present invention can be widely used inthe production of, for example, sealing rubber products such as an0-ring, a packing or a gasket used for a bearing of a rotating device;belts such as a conveyor belt, a V-belt or a timing belt; valves andvalve-sealing materials; oil well packers, well-head sealing materials,BOP (blow-out preventers) and bladders; cushioning materials andvibration insulators; ship and automobile bearing seals such as crankshaft seals, bearing seals, rotary seals of accelerators and stern tubeseals; various diaphragms; and hoses such as automobile fuel hoses,marine hoses, riser hoses and flow line hoses, and further, can be usedin the energy field, for example, as articles for geothermal electricpower generation.

The present invention will now be described in detail with reference tothe following examples. Note, in the examples, comparative examples andreferential examples, all of "parts" and "%" are by weight unlessotherwise indicated.

EXAMPLES 1 THROUGH 10 AND COMPARATIVE EXAMPLES 1 THROUGH 10

An acrylonitrile/butadiene copolymer rubber oracrylonitrile/butadiene/fluorine-free unsaturated carboxylic acid estermonomer terpolymer rubber, prepared by a customary emulsionpolymerization, was dissolved in methyl isobutyl ketone, and thebutadiene units of the rubber were partially hydrogenated in apressure-resistant vessel by using a Pd/silica catalyst, to obtain ahighly saturated copolymer rubber. The contents of the monomer units inthe rubber are shown in Table 1.

The obtained highly saturated copolymer rubber was then mixed withadditives shown in Table 2 on a cooling roll, to obtain a rubbercomposition, and the rubber composition was heated under pressure at170° C. for 15 minutes to obtain a cured product. The TR test of thecured product was carried out according to ASTM D-1329, and the otherphysical properties of the cured product were measured according to JIS(Japanese Industrial Standard) K-6301. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                    Examples of the Invention                                                     1  2  3  4  5  6  7  8  9  10                                 __________________________________________________________________________    Monomer units (%):                                                            Acrylonitrile   15 14 14 15 15 20 20 20 20 20                                 Diethyl itaconate                                                                             42 -- -- -- -- 37 -- -- -- --                                 Di-n-butyl itaconate                                                                          -- 45 -- -- -- -- 38 -- -- --                                 Diethylaminoethyl methacrylate                                                                -- -- 45 -- -- -- -- 39 -- --                                 Butyl acrylate  -- -- -- 43 -- -- -- -- 35 --                                 2-Ethylhexyl acrylate                                                                         -- -- -- -- 45 -- -- -- -- 38                                 Butadiene       5  5  13 7  5  5  5  6  10 8                                  Hydrogenated butadiene                                                                        38 36 28 35 35 38 37 35 35 34                                 Physical properties in                                                        normal state:                                                                 Tensile strength (kg/cm.sup.2)                                                                186                                                                              190                                                                              182                                                                              210                                                                              213                                                                              214                                                                              203                                                                              190                                                                              210                                                                              224                                Elongation (%)  230                                                                              250                                                                              530                                                                              250                                                                              240                                                                              370                                                                              240                                                                              380                                                                              230                                                                              270                                Hardness (points)                                                                             61 58 57 61 62 61 61 57 60 64                                 Change in physical properties                                                 after heat aging at 150° C. for                                        72 hours:                                                                     Change ratio of tensile                                                                       -5 -12                                                                              -34                                                                              -18                                                                              -11                                                                              -1 -20                                                                              -24                                                                              -16                                                                              -15                                strength (%)                                                                  Change of elongation (%)                                                                      -15                                                                              -12                                                                              -30                                                                              -20                                                                              -25                                                                              -12                                                                              -21                                                                              -23                                                                              -22                                                                              -19                                Change of hardness                                                                            -4 -2 -2 -1 -2 -3 -1 -1 -1 -1                                 TR-10 (°C.)                                                                            -35                                                                              -39                                                                              -41                                                                              -30                                                                              -26                                                                              -33                                                                              -35                                                                              -38                                                                              -27                                                                              -26                                                Comparative Examples                                                          1  2  3  4  5  6  7  8  9  10                                 __________________________________________________________________________    Monomer units (%):                                                            Acrylonitrile   15 14 14       20 20 45 20 34                                 Diethyl itaconate                                                                             10 -- --    -- -- 10 -- --                                    Di-n-butyl itaconate                                                                          -- 10 --       5  -- -- -- --                                 Diethylaminoethyl methacrylate                                                                -- -- 10       -- 5  -- -- --                                 Butyl acrylate  -- -- --       -- -- -- -- --                                 2-Ethylhexyl acrylate                                                                         -- -- --       -- -- -- -- --                                 Butadiene       5  5  13       10 10 5  5  5                                  Hydrogenated butadiene                                                                        70 71 63       65 65 40 75 61                                 Physical properties in                                                        normal state:                                                                 Tensile strength (kg/cm.sup.2)                                                                210                                                                              200                                                                              200      220                                                                              210                                                                              230                                                                              190                                                                              265                                Elongation (%)  270                                                                              280                                                                              380      290                                                                              350                                                                              300                                                                              300                                                                              430                                Hardness (points)                                                                             61 59 63       60 62 62 65 65                                 Change in physical properties                                                 after heat aging at 150° C. for                                        72 hours:                                                                     Change ratio of tensile                                                                       -7 -15                                                                              -35      -12                                                                              -32                                                                              -10                                                                              -8 -6                                 strength (%)                                                                  Change of elongation (%)                                                                      -20                                                                              -20                                                                              -19      -17                                                                              -26                                                                              -15                                                                              -10                                                                              -7                                 Change of hardness                                                                            +1 +2 +3       +1 +2 +1 +2 +5                                 TR-10 (°C.)                                                                            -18                                                                              -22                                                                              -15      -15                                                                              -17                                                                              -17                                                                              -10                                                                              -24                                __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                        (Recipe)                                                                      ______________________________________                                        Highly saturated copolymer rubber                                                                     100    parts                                          Zinc oxide              5      parts                                          Stearic acid            1      part                                           SRF carbon black        40     parts                                          Peroximon F40.sup.(1)   5      parts                                          ______________________________________                                    

NOTE

(1) m,p-di-isopropylbenzene α,60 '-bis-t-butyl

peroxide supplied by Montecatini (Italy)

As apparent from Comparative Examples 1 through 8 in Table 1, the coldresistance (TR10) of a copolymer rubber containing unsaturatedcarboxylic acid ester units which fails to satisfy the requirements ofthe present invention, is not improved.

EXAMPLES 11 THROUGH 21 AND COMPARATIVE EXAMPLES 11 THROUGH 14

A rubber composition was prepared according to a recipe shown in Table 2by using a highly saturated copolymer rubber shown in Table 3, which wasprepared in the same manner as described in Example 1, and the rubbercomposition was heated under pressure at 170° C. for 15 minutes toobtain a cured product having the properties shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                                                     Comparative                                Examples of the Invention          Examples                                   11  12 13 14 15 16  17 18 19 20 21 11  12 13 14                 __________________________________________________________________________    Monomer units (%):                                                            Acrylonitrile 23  28 34 22 28 23  34 25 32 24 31 21  25 24 25                 Trifluoroethyl acrylate                                                                     20  15 10 32 23 --  -- -- -- -- -- 5   -- -- --                 Trifluoroethyl methacrylate                                                                 --  -- -- -- -- 22  10 -- -- -- -- --  -- -- --                 Tetrafluoropropyl acrylate                                                                  --  -- -- -- -- --  -- 30 19 -- -- --  3  -- --                 Octafluoropentyl acrylate                                                                   --  -- -- -- -- --  -- -- -- 30 20 --  -- 3  --                 Butadiene     10  10 10 5  5  5   10 5  5  5  5  5   5  5  5                  Hydrogenated butadiene                                                                      47  47 46 41 44 50  46 40 44 41 44 69  67 68 70                 Physical properties in normal                                                 state:                                                                        Tensile strength (kg/cm.sup.2)                                                              223 210                                                                              238                                                                              243                                                                              263                                                                              206 246                                                                              224                                                                              254                                                                              210                                                                              245                                                                              200 230                                                                              240                                                                              270                Elongation (%)                                                                              290 260                                                                              260                                                                              220                                                                              300                                                                              280 320                                                                              240                                                                              280                                                                              220                                                                              280                                                                              290 290                                                                              300                                                                              260                Hardness (points)                                                                           66  65 65 65 66 64  71 70 70 70 64 63  65 64                    Change in physical properties                                                 after heat aging at 150° C. for                                        72 hours:                                                                     Change ratio of tensile                                                                     -26 -32                                                                              -12                                                                              -14                                                                              -8 -31 -6 -9 -8 ±0                                                                            -8 -10 -16                                                                              -20                                                                              -20                strength (%)                                                                  Change of elongation (%)                                                                    -23 -25                                                                              -20                                                                              -14                                                                              -13                                                                              -29 -22                                                                              -12                                                                              -21                                                                              -14                                                                              -21                                                                              -13 -24                                                                              -20                                                                              -22                Change of hardness                                                                          +2  +1 +2 +2 +2 +2  +2 +1 +3 +2 +3 +3  +5 +4 +6                 TR-10 (°C.)                                                                          -28 -29                                                                              -30                                                                              -30                                                                              -30                                                                              -28 -26                                                                              -30                                                                              -29                                                                              -30                                                                              -29                                                                              -15 -19                                                                              -18                                                                              -14                __________________________________________________________________________

EXAMPLE 22 AND COMPARATIVE EXAMPLE 15

Highly saturated copolymer rubbers having the monomer units shown inTable 4 were prepared in the same manner as described in Example 1 byusing a butadiene/isoprene/butyl acrylate/acrylonitrile copolymer(composition ratio =52/10/10/28) and a butadiene/isoprene/acrylonitrilecopolymer rubber composition ratio =62/10/28) for comparison.

Rubber compositions were obtained by mixing the copolymer rubbers withadditives shown in Table 5 on a roll, the rubber compositions wereheated under pressure at 160° C. for 20 minutes to obtain curedproducts, and the characteristic properties thereof were measured. Theresults are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                                Comparative                                                            Example                                                                              Example                                                                22     15                                                    ______________________________________                                        Monomer units (%):                                                            Acrylonitrile      28       28                                                Butyl acrylate     10       --                                                Diene              11       11                                                Hydrogenated diene 51       61                                                Physical properties in normal                                                 state:                                                                        Tensile strength (kg/cm.sup.2)                                                                   239      252                                               Elongation (%)     240      300                                               Hardness (points)  64       63                                                Change in physical properties                                                 after heat aging at 150° C. for                                        72 hours:                                                                     Change ratio of tensile                                                                          -16      -17                                               strength (%)                                                                  Change of elongation (%)                                                                         -28      -29                                               Change of hardness (points)                                                                      +4       +5                                                TR-10 (°C.) -30      -21.5                                             ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        (Recipe)                                                                      ______________________________________                                        Highly saturated copolymer rubber                                                                     100    parts                                          Zinc oxide              5      parts                                          Stearic acid            1      part                                           Sulfur                  0.5    part                                           SRF carbon black        40     parts                                          Tetramethylthiuram disulfide                                                                          2      parts                                          2-Mercaptobenzothiazole 0.5    part                                           ______________________________________                                    

EXAMPLE 23 THROUGH

Each of the hydrogenated to polynmer rubbers of the present inventionobtained in Example 1 through 21 was mixed with the additives shown inTable 5. The obtained rubber compositions were heated under pressure at160° C. for 20 hours, and the TR10 of each of the obtained curedproducts was measured. The results were substantially the same as theresults obtained in Examples 1 through 21.

We claim:
 1. A nitrile group-containing, highly saturated copolymerrubber, the copolymer chain of which comprises, based on the weight ofthe copolymer chain, (1) 5 to 40% by weight of units of an unsaturatednitrile monomer, (2) 15 to 80% by weight of units of a mono- or dialkylester of an unsaturated dicarboxylic acid having 1 to 8 carbon atoms inthe alkyl group and (3) up to 20% by weight of units of a conjugateddiene monomer, with the balance being (4) units of a hydrogenatedconjugated diene monomer, wherein the sum of the contents of the monomerunits (1) and (2) is 30 to 90% by weight and the sum of he contents ofthe monomer units (3) and (4) is 10 to 70% by weight.
 2. The copolymerrubber according to claim 1 wherein the unsaturated nitrile monomer isselected from the group consisting of acrylonitrile, methacrylonitrileand 60 -chloroacrylonitrile.
 3. The copolymer rubber according to claim1 wherein the amount of the monomer units (1) is 10 to 35% by weight,the amount of the monomer units (2) is 15 to 60% by weight, and the sumof the amounts of the monomer units (1) and (2) is 40 to 90% by weight.4. The copolymer rubber according to claim 1 wherein the conjugateddiene monomer is selected from the group consisting of butadiene andisoprene.
 5. The copolymer rubber according to claim 1 wherein theamount of the monomer units (3) is up to 15% by weight and the sum ofthe amounts of the monomer units (3) and (4) is 10 to 60% by weight. 6.A rubber composition comprising 25 to 90% by weight, based on the rubbercomposition, of a nitrile group-containing, highly saturated copolymerrubber and 75 to 10% by weight, based on the rubber composition, ofadditives; the copolymer chain of said nitrile group containing, highlysaturated copolymer rubber comprising, based on the weight of thecopolymer chain, (1) 5to 40% by weight of units of an unsaturatednitrile monomer, (2) 15 to 80% by weight of units of a mono- or di-alkylester of an unsaturated dicarboxylic acid having 1 to 8 carbon atoms ineach alkyl group and (3) up to 20% by weight of units of a conjugateddiene monomer, with the balance being (4) units of a hydrogenatedconjugated diene monomer, wherein the sum of the contents of the monomerunits (1) and (2) is 30 to 90% and the sum of the contents of themonomer units (b 3) and (4) is 10 to 70% by weight.
 7. The rubbercomposition according to claim 6 wherein the unsaturated nitrile monomeris selected from the group consisting of acrylonitrile,methacrylonitrile and α-choloracrylonitrile.
 8. The rubber compositionaccording to claim 6 wherein the amount of other monomer units (1) is 10to 35% by weight, the amount of the monomer units (2) is 15 to 60% byweight, and the sum of the amounts of the monomer units (1) and (2) is40 to 90% by weight.
 9. The rubber composition according to claim 6wherein the conjugated diene monomer is selected rom the groupconsisting of butadiene and isoprene.
 10. The rubber compositionaccording to claim 6 wherein the amount of the monomer units (3) is upto 15% by weight and the sum of the amounts of the monomer units (3) and(4) is 10 to 60% by weight.
 11. The copolymer rubber according to claim1, wherein the unsaturated dicarboxylic acid ester is selected from thegroup consisting of dialkyl esters of unsaturated dicarboxylic acids,wherein each alkyl group has 1 to 8 carbon atoms.
 12. The copolymerrubber according to claim 6, wherein the unsaturated dicarboxylic acidester is selected from the group consisting of dialkyl esters ofunsaturated dicarboxylic acids, each alkyl group having 1 to 8 carbonatoms.